1. Field of the Invention
The present invention relates to an optical cross-connect device that is capable of changing paths for optical signals, viz. switching, addressing and cross-connecting a plurality of input light signals to a plurality of outlet optical waveguides directly without converting the optical signals into electrical signals, in a WDM (Wavelength Division Multiplexing) transmission control apparatus that permits high-speed signal transmission in an optical backbone network.
2. Description of the Prior Art
With the tremendous spreads of personal computers and cellular phones for use to access Internet sites, high speed broadband communication services by xDSL and sustained connection options have come into wide use in the communication infrastructure as well. And, to harmonize with the next-generation Internet communication and the next-generation cellular phone a further drastic rise in the line speed in the backbone network is being sought after.
In transmitting signals on optical-fiber cables, the conventional TDM (time division multiplexing) system permits light signals of only one wavelength to be transmitted on one optical-fiber cable. In contrast the WDM (wavelength division multiplexing) system allows light signals of a number of different wavelengths to be transmitted on one optical-fiber cable, and hence permits increasing the bands and elevating the line speed to a large extent with the use of the established optical-fiber cables. In a WDM transmission apparatus using such a WDM technique, switching paths for input light signals and paths for output light signals, that is, cross-connecting them, rapidly makes it essential to include an optic switch that is capable of directly switching those optical paths mutually without the need to convert the input light signals into electric signals and the latter to the output light signals.
So far, four types of such optic switches have been proposed. The first is what is called the “mirror type switch” in which a small mirror formed of a Si substrate is either rotated or vertically moved to cause an incident light to be reflected at a specified angle to change its path. The second is what is called the “planar optical waveguide type switch” that uses a waveguide having a crossover in which a heater is embedded, the heater being heated up to change its refractive index for an incident light, thereby changing its path. The third is what is called the “bubble type switch” in which the heater is replaced by a groove filled with a liquid so that the generation of bubbles in the groove may be controlled to change the angle of reflection of an incident light, thereby changing its path. The fourth is what is called the “mechanical type switch” in which an optical-fiber cable is bent, e.g., with an electromagnet, to change the path of an incident light.
Of these prior types of optic switches, the “mirror type switch” is considered the largest in scale at this point of time inasmuch as it can provide channels reaching several hundreds in number. However, this switch requiring the movement of a small mirror that must be rotated or vertically moved has a problem in durability. It also has the problem that its switching speed is as low as requiring 50 milliseconds for switching. On the other hand, the “mechanical type switch” is operable at a switching speed as relatively high as can be switched in 10 milliseconds or less, but has the problem that it can only provide several channels and hence cannot be utilized but for limited, small scale applications. The “planar optical waveguide type switch” and the “bubble type switch” are also operable at a switching speed as relatively high as can be switched in 10 milliseconds or less, and can provide channels which are several tens in number. However, using the structure requiring that a groove filled with a heater or liquid be formed in a substrate, these switches have left more or less to be desired in durability. Further, while no mechanical movement is required in these switches, they do require more or less mechanical components, presenting the problem that their structures become intricate.